Liposomes are microscopic vesicles composed of a single phospholipid bilayer or a plurality of concentric phospholipid bilayers which enclose aqueous phases. These vesicles can serve as drug carriers for hydrophobic as well as hydrophilic substances, including various drug molecules and peptides. Liposomes are prepared mainly from phospholipids of different headgroups and chain lengths, and cholesterol. Due to physicochemical similarities with cell membranes, liposomes are biocompatible, biodegradable and non-toxic vehicles for drug molecules.
U.S. Pat. Nos. 4,911,928 and 5,234,767, both to Wallach, describe lipid vesicles as substantially spherical structures made of materials having high lipid content, e.g. surfactant or phospholipid; (see column 1 lines 25 to 27 of U.S. Pat. No. 5,234,767). The lipids of these spherical vesicles are said to be organized in the form of lipid bilayers. The lipid bilayers encapsulate an aqueous volume which is either interspersed between multiple onion-like shells of lipid bilayers (forming multilamellar lipid vesicles or "MLV") or the aqueous volume is contained within an amorphous central cavity. Mention is also made of large unilamellar vesicles ("LUV") generally having a diameter greater than about 1 .mu.m, small unilamellar vesicles ("SUV") generally having a diameter less than about 0.2 .mu.m and paucilamellar lipid vesicles ("PLV") which have about 2 to about 8 or 10 peripheral bilayers. Although Wallach says that PLV's can be considered to be a sub-class of the MLV's, he says that electron micrographs confirm that the paucilamellar lipid vesicles are distinct from the LUV's and the classic MLV's; (U.S. Pat. No. 4,911,928 column 3, lines 29 to 31) This invention is not concerned with PLV's but is concerned with classic MLV's having substantially more lipid bilayers than PLV's, the MLV's having at least about 15 lipid bilayers, normally more than about 30 and frequently over 100.
Liposomes are prime candidates for the systemic as well as topical delivery of drugs. Several studies showed that liposome encapsulation advantageously alters the pharmacokinetic fate of the drug after topical application. The first experimental evidence that liposomes may penetrate into the skin and deposit within the dermis was obtained recently by using a novel small particle-size electrondense marker (colloidal iron), which can be efficiently encapsulated into liposomes of various sizes and compositions, and can be easily identified in cells and tissues by electron microscope.
However, current liposome technology suffers from a number of important drawbacks that has prevented its widespread use in dermal applications and in systemic delivery. Hence, liposome formulations present consistency problems for skin applications that have so far been solved by either adding viscosity-increasing agents such as cellulose derivatives to the pharmaceutical composition that contains the liposomes as active ingredient or by using higher concentrations of phospholipids in the preparation of the liposomes.
The addition of viscosity-increasing agents to the final liposome composition usually dilutes the product but, more importantly, these agents were shown to decrease the release of encapsulated drug from the liposomes; Foldvari, et al. (1993) J. Controlled Release 27; 193-205. Also, viscosity-increasing agents have a detrimental effect on the integrity of the liposomes. With regard to phospholipids, lipids, they are the main and also the most expensive liposome-forming ingredient. Their use as a viscosity-increasing agent is therefore not economically practical.
Two potential applications of the liposome technology described herein are the treatment of sexual dysfunction and the treatment of genital warts.
1. Sexual Dysfunction
Impotence, the partial or complete inability of the male to perform the sexual act or to achieve orgasm, may be psychogenic and/or organic in nature. Initially, psychogenic etiology was considered to account for 95 percent of cases of impotence but it is now apparent that organic factors are almost invariably involved in cases of impotence. Thus, impotence is best characterized as either of predominantly organic or predominantly psychogenic etiology.
Impotence is a socially crippling disease. It detracts from the patient's self esteem and in some cases his ability to interact with others. Current therapy modes include psychotherapy, microsurgery (to restore compromised vascular supply), implantation of penile prosthesis and pharmacotherapy.
Numerous agents have been employed in the therapy of impotence. Hormonal manipulation, such as replacement androgen therapy, has been used. However, patients with organic impotence are rarely candidates for hormonal manipulation. Thus, this mode of therapy should be strictly limited to cases in which endocrine deficiencies are established. Furthermore, in about 50 percent of patients with an endocrinopathy, a psychogenic etiology can be shown to be the predominant factor in erectile difficulties. Therefore, in the vast majority of impotent men, vascular and neurologic factors are the underlying causes. The most commonly used treatment for these patients is the implantation of penile prostheses. The invasive nature of this technique, coupled with the increasing evidence of mechanical failure, surgical complications, or infection has again focused attention on the development of pharmacological agents with a potential for improving the libido and quality of erections. Several agents have been tested e.g. bromocriptine, glyceryl trinitrate, zinc, oxytocin, yohimbine and nitroglycerin. Intracorporeal injection of vasoactive agents is now considered the treatment of choice for many patients with organic impotence (Kattan et. al., 1991).
Injection of papaverine, a smooth muscle relaxant, or a combination of papaverine and phentolamine (an a-adrenergic blocker) directly into the corpus cavernosum (either of the two erectile columns of the dorsum of the penis) has been shown to be an effective therapy in impotence. Recently, intercavernosal injections of prostaglandin E.sub.1 (alprostadil), a naturally occurring chemical derived from dihomo-gammalinolenic acid (20:3.infin.6) was discovered to also induce erection.
Injection of prostaglandin E.sub.1 results in vasodilation, with increased arterial inflow and decreased venous outflow by occlusion of draining venules, probably through relaxation of corporal smooth muscle. Due to its potent relaxant effect on vascular smooth muscle, prostaglandin E.sub.1 is used to maintain the patency of the ductus arteriosus in the neonate. This is the only currently approved (FDA) indication for prostaglandin E.sub.1. The effect of prostaglandin E.sub.1 on erectile dysfunction is known to be dose dependent.
Prostaglandin E.sub.1 (PGE.sub.1), a potent relaxant of the vascular smooth muscle, has been shown to be effective and safe in the treatment of impotence by increasing arterial inflow through vasodilatation and decreasing venous outflow by the occlusion of draining venules due to relaxation of corporal smooth muscle. As opposed to other previously used drugs such as papaverine or phentolamine, PGE.sub.1 is not as frequently associated with the common side effects e.g. priapism, plaques at the injection site and liver function abnormalities, therefore it is clinically more acceptable. PGE.sub.1 is usually self-injected into the corpus cavernosum through the lateral aspect of the shaft of the penis. This type of administration, however, is associated with penile discomfort, pain at the injection site and the inconvenience of application prior to intercourse. Topical application of PGE.sub.1 would be an ideal route of administration, however, the drug itself (without a delivery system) cannot penetrate the skin in adequate concentration and would be metabolized within the skin very quickly before reaching the underlying tissues.
2. Genital Warts
It is estimated that condylomata acuminata contributes to 1.3-1.4 million office visits per year in the United States, representing a 580% increase between 1966 and 1983 (Center for Disease Control, 1983). Condylomata acuminata is caused by the human papillomavirus (HPV) and it is one of the most commonly diagnosed viral sexually transmitted diseases. More than 50 types of HPV has been categorized and new types are constantly being identified. Certain subsets of HPV (mostly type 16 and 18) has been shown to be associated with malignant tumors of the ano-genital tract, respiratory tract and the skin.
Conventional therapy for genital warts has included podophyllin, cryotherapy, trichloroacetic acid, electrical cautery, laser ablation and conventional surgery, and has been directed primarily at visible lesions. Recurrences after therapy for HPV infections is very common, which could be due to incomplete eradication of the virus since the presence of residual virus in normal-appearing tissue has been demonstrated.
Interferon has activity against papillomaviruses, and cures infected cells by eliminating extrachromosomal viral DNA. Systemic application (i.m. injection) of interferon alpha in patients with genital warts was shown to be fairly successful, however it is associated with various side effects such as fever, myalgias, headache, nausea, fatigue. Intralesional IFN treatment appears to be a more promising approach for visible lesions, but it is not suitable for latent or subclinical infections. Initially highly positive results (90% complete response) were reported with topical natural leukocyte IFN. Vesterinen et al. reported colposcopic remission in five out of eight patients with vaginal flat condylomas treated with a potent topical IFN cream. However, in spite of the improvement of clinical appearance, the cytology remained positive in all cases and two responding patients had recurrences in two months.